Composition Useful in Sulfate Scale Removal

ABSTRACT

The present invention discloses a novel aqueous composition for use in removing mixed sulfate scale from a surface contaminated with such, said composition comprising: a chelating agent and a counterion component selected from the group consisting of: Li5DTPA; Na5DTPA; K5DTPA; Cs5DTPA; Na4EDTA; K4EDTA; TEAH4DTPA; and TBAH5DTPA; a dissolution enhancer; optionally a compound such as sodium gluconate or the like and a carboxyl-containing fructan such as carboxymethyl inulin. There is also disclosed methods to use such compositions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to CanadianApplication No. 3,049,343, filed Jul. 11, 2019. The entire specificationand figures of the above-referenced application are hereby incorporated,in their entirety by reference.

FIELD OF THE INVENTION

The present invention is directed to a composition and method for use inoilfield and industrial operations, more specifically to compositionsused in the removal of mixed sulfate scale.

BACKGROUND OF THE INVENTION

Scaling, or the formation of sulfate mineral deposits can occur onsurfaces of metal, rock or other materials. Scale is caused by aprecipitation process as a result of thermodynamic and hydrodynamicfactors or changes in pressure, velocity rates and temperature and thesubsequent change in the composition of a solution (commonly water).

Typical homogenous scales consist of e.g. calcium carbonate, calciumsulfate, barium sulfate, strontium sulfate, iron sulfide, iron oxides oriron carbonate. Sulfate scales and in particular the common Bariumsulfate scale is a major challenge for industry and in particular theoilfield industry.

In some cases, scale deposits restrict or even shut-off the productionconduit if the produced water composition flow dynamics are interruptedby changes in pressure and/or temperature. In many cases this is due towellbore components, such as downhole chokes, safety valves orflow-controls. In addition to produced formation brine water scalingissues due to the mineral content, also other sourced water utilized inwell workover or completions operations can be potential sources ofscaling minerals, including water flood operations or geothermaloperations.

The precipitation of sulfate scales can occur at any point in aproduction, injection or abandonment-disposal well and associateequipment cycle, and can also be caused by incompatibilities of injectedwater and formation water, in addition to the changes in temperature andpressure dynamics mentioned above, as well as wellbore additives orupsets in the flow equilibrium. Scale on surface equipment (e.g. heatexchangers, piping, valves, flow-control devices) are also a catalystfor sulfate scales. In oil & gas operations, seawater or brine is ofteninjected into reservoirs for pressure maintenance, and as these have ahigh content of sulfate ions and formation water or drilling fluidsoften have a high content of barium, calcium, and/or strontium ionsdonated from the formation, these waters often cause sulfate mineralprecipitation. Sulfate scaling on surface equipment, such as heatexchangers and the associated piping, is a major issue for industry aswell, as it typically needs to be managed by mechanical means such asdisassembling the equipment in question, manually cleaning the scale andreassembling which is very time consuming and expensive. Having achemical solution that can treat these sulfate scales with minimalagitation and at lower temperatures would be very advantageous forindustry. As the multiple sulfate composition scaling challenges occuroffshore-onshore are typically very difficult to manage efficiently as awhole. Having a sulfate dissolver that solubilizes all typical sulfatescales encountered either individually or as a composition isadvantageous for industry versus having to deploy specific chemistry foreach type or scale or manage the scaling issues with mechanical means.

The most obvious way of preventing a scale from forming duringproduction is to prevent the creation of super saturation of the brinebeing handled and manage the flow path of fluids to minimize pressureand rate differentials and to also add scale inhibitors, whichthemselves are minimally effective and expensive. It may also sometimesbe possible by altering the operating conditions of the reservoir, forexample by ensuring that the wellbore pressure is sufficient to preventthe liberation of gas and by injecting water which is compatible withformation water. However, economics usually dictate that the use ofinhibitors or treating any precipitated scale is preferred to managecosts

Controlling scale by the use of inhibitors as well as understanding andmitigating scaling tendencies is important for both production andinjection wells, but so also is having a solution or economical means oftreating any scaling that does occur, even after best practices havebeen implemented during the production cycle.

The design of scale treatment programs requires extensive knowledge ofscaling/chemistry theory and a broad base of practical operationalexperience to be successful. Applications occasionally presentthemselves in which the ideal selection of chemicals and fluids may bebeyond the scope of a wellsite engineer's experience or theoreticalknowledge. Rules of thumb and general formulas may not be adequate, andselection procedures based on broader experience and more in-depthknowledge may be required. Analysis of deposits and dissolver screeningideally when considering a potential scale dissolving application, thescale that is causing the problems will have to be analyzed.

The most common sulfate scales are barium, calcium, and strontium. Thesealkaline earth metal salts have many similar properties and oftenprecipitate together forming problematic sulfate scales. The depositionof this scale is a serious problem for oil and gas producers globally,causing fouling in the wellbore and surface related processing equipmentThese scales not only restrict the hydrocarbon flow from the formationresulting in lost production, and since the formation or injection wateris saturated with sulfates, the continued deposition causes fouling andpotentially failures of critical equipment such as perforations, casing,tubes, valves, and surface equipment, all with the potential to reducethe rate of oil production and result in substantial lost revenue. Thereis a need in industry for an effective solution to an ongoing challenge.Sulfate scales such as radium sulfate, barium sulfate, calcium sulfateetc.—are sometimes referred to as NORM scale due to their solubilitycharacteristics—typically 0.0023 g/l in water—are more difficult to dealwith than carbonate scales. Sulfate scales are not soluble intraditional acid scale dissolvers. Radium sulfate, while not being themost common sulfate scale represents a challenge in its removal as it isoften imbedded in barium sulfate scale and is also radioactive and thuscan carry an exposure risk and cause very expensive clean-up or disposalcosts of tubing and down-hole equipment etc. when brought out of thewell for replacement, general service or abandonment. Having a chemicalthat can be used to wash these components while still in the well andeffectively clean/remove the NORM materials leaving them down-hole,allowing the operator to greatly reduce handling/disposal costs relatedto NORM containing wells is very advantageous.

Once this water/acid insoluble scale has formed, it is extremelydifficult to remove with existing chemical options on the market.

The solubility of barium sulfate is reported to be approximately0.0002448 g/100 ml (20° C.) and 0.000285 g/100 ml (30° C.). Existingmethods to remove sulfate scale include mechanical removal and/or lowperformance scale dissolvers currently on the market, but both havelimitations and disadvantages. Mechanical removal involves the use ofmilling tools, scraping, or high-pressure jetting and/or disassembly ofkey production equipment causing substantial down time of production andprocessing equipment. These methods have limited efficiency as the scaleis extremely hard to remove, often forming in areas beyond the reach ofthe mechanical equipment as many facilities have welded joints andlimited access. High pressure jetting will typically only remove thesurface of the scale.

Sulfate scale dissolvers were developed to overcome the low solubilityof these types of scale. Sulfate scale dissolvers work bychelating/mopping up the dissolved sulfate that is present in the waterallowing more to be dissolved. To assist the rate of reaction/increasethe speed and efficiency of dissolution these products are typicallydeployed at elevated temperatures of 50° C. to 90° C. Sulfate scaledissolution will as a result take far longer than for example carbonatescale dissolution utilizing and acid. Typical scale dissolvers such asethylenediaminetetraacetic acid (EDTA), and variations of this molecule(such as DTPA) are used by the industry to dissolve sulfate scale withsome success, and sequestering the barium, calcium, and strontium ions.However, this process requires higher temperature (usually above 75°C.), is time-consuming, and has limited dissolution capacity.

The following include some patent disclosures of sulfates scaleremovers. U.S. Pat. No. 4,980,077 A teaches that alkaline earth metalscales, especially barium sulfate scale deposits can be removed fromoilfield pipe and other tubular goods with a scale-removing compositioncomprising an aqueous alkaline solution having a pH of about 8 to about14, a polyaminopolycarboxylic acid, preferably EDTA or DTPA and acatalyst or synergist comprising oxalate anion. It is stated that whenthe scale-removing solution is contacted with a surface containing ascale deposit, substantially more scale is dissolved at a faster ratethan previously possible.

WO 1993024199 A1 teaches the use of low frequency sonic energy in thesonic frequency range for enhancing the dissolution of alkaline earthmetal scales using a scale-removing solvent comprising an aqueousalkaline solution having a pH of about 8 to about 14 and containing EDTAor DTPA and a catalyst or synergist, preferably an oxalate anion. It isstated that when the scale-removing solvent is contacted with a surfacecontaining a scale deposit while simultaneously transmitting lowfrequency sonic energy through the solvent, substantially more scale isdissolved at a faster rate than previously possible.

U.S. Pat. No. 4,030,548A teaches a barium sulfate scale or solid can bedissolved economically by flowing a stream of relatively dilute aqueoussolution of aminopolyacetic acid salt chelating agent into contact withand along the surfaces of the scale while correlating the compositionand flow rate of the solution so that each portion of solution containsan amount of chelant effective for dissolving barium sulfate and theupstream portions of the scale are contacted by portions of the solutionwhich are unsaturated regarding the barium-chelant complex.

U.S. Pat. No. 6,613,899 B1 teaches carboxyl-containing fructans such ascarboxymethylinulin used to prevent deposition of scale composed of, forexample, calcium, barium and strontium salts of sulphuric acid andcarbonic acid in oil extraction. In the oil extraction method, 0.5-200ppm of a carboxyl-containing fructan that contains 0.3-3 carboxyl groupsper mono-saccharide unit is incorporated in the process water, in theprocess equipment or in the oil-containing formation.

U.S. Pat. No. 3,625,761A teaches a method of removing a deposit ofalkaline earth metal sulfate scale in an aqueous system which comprisescontacting said scale deposit with a treating composition heated to atemperature in the range of from about 86 to about 194° F. consistingessentially of an aqueous alkaline solution containing from about 4 toabout 8 percent by weight of disodium hydrogenethylenediaminetetraacetate dihydrate and having a pH in the range ofabout 10 to 13 for a period sufficient to dissolve at least some of thesaid scale, acidifying said solution to decrease the pH thereof to a pHin the range of from 7 to 8 with an acid selected from the groupconsisting of sulfuric acid, hydrochloric acid, oxalic acid, a mixtureof sulfuric acid and oxalic acid, and a mixture of hydrochloric acid andoxalic acid, to precipitate any alkaline earth metal ion present.

U.S. Pat. No. 5,084,105A teaches that alkaline earth metal scales,especially barium sulfate scale deposits can be removed from oilfieldpipe and other tubular goods with a scale-removing compositioncomprising an aqueous alkaline solution having a pH of about 8 to about14, preferably about 11 to 13, of a polyaminopolycarboxylic acid,preferably EDTA or DTPA and a catalyst or synergist comprising amonocarboxylic acid, preferably a substituted acetic acid such asmercaptoacetic, hydroxyacetic acid or aminoacetic acid or an aromaticacid such as salicylic acid. The description states that when thescale-removing solution is contacted with a surface containing a scaledeposit, substantially more scale is dissolved at a faster rate than ispossible without the synergist.

U.S. Pat. No. 7,470,330 B2 teaches a method of removing metal scale fromsurfaces that includes contacting the surfaces with a first aqueoussolution of a chelating agent, allowing the chelating agent to dissolvethe metal scale, acidifying the solution to form a precipitant of thechelating agent and a precipitant of the metal from the metal scale,isolating the precipitant of the chelating agent and the precipitant ofthe metal from the first solution, selectively dissolving theprecipitated chelating agent in a second aqueous solution, and removingthe precipitated metal from the second solution is disclosed. This isunderstood to be a multi-step process which would cause longer shutdownin production and is not determined to actually be applicable in thefield.

Despite the existing prior art, there are very few commercialcompositions available to remove barium sulfate scale, the situation ismade even more complex since most barium sulfate scale occurs inwellbores, pipes and other equipment associated with either oilproduction and/or oil exploration in offshore or highly regulatedjurisdictions such as the North Sea. Thus, the removal ofpetroleum-contaminated barium sulfate scales presents an even morechallenging task for oilfield operators. it is highly advantageous toindustry to have a chemical option that meets these stringentenvironmental and HSE parameters.

There thus still exists a profound need for compositions and methodscapable of removing very difficult to remove mixed sulfate scalespresent in oilfield equipment.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided amethod of removing mixed sulfate scale, said method comprising:

providing a liquid composition comprising:

a chelating agent selected from the group consisting of: Li₅DTPA;Na₅DTPA; K₅DTPA; Cs₅DTPA; Na₄EDTA; K₄EDTA; TEAH₄DTPA; and TBAH5DTPA;

optionally, a scale removal enhancer;

a carboxyl-containing fructan such as carboxymethyl inulin; and

a compound selected from the group consisting of: sodium gluconate;gluconic acid; glucono-delta-lactone; sodium gluconate; calciumgluconate; potassium gluconate;

exposing a surface contaminated with said mixed sulfate scale to theliquid composition;

allowing sufficient time of exposure to remove said mixed sulfate scalefrom the contaminated surface.

Preferably, the scale removal enhancer is selected from the groupconsisting of: potassium carbonate; potassium formate; cesium formate;cesium carbonate; and combinations thereof.

Preferably, the carboxyl-containing fructan is a derivative of inulin oranother fructan that contains 0.3-3 carboxyl groups per anhydrofructoseunit. Preferably, the derivative of inulin or another fructan thatcontains 0.3-3 carboxyl groups per anhydrofructose unit contains atleast 0.8 carboxyl groups per anhydrofructose unit. More preferably, thecarboxyl-containing fructan is carboxymethylinulin (CMI).

According to an aspect of the present invention, there is provided anaqueous composition for use in removing mixed sulfate scale from asurface contaminated with such, said composition comprising:

a chelating agent and a counterion component selected from the groupconsisting of: Li₅DTPA; Na₅DTPA; K₅DTPA; Cs₅DTPA; Na₄EDTA; K₄EDTA;TEAH₄DTPA; and TBAH₅DTPA;

optionally, a scale removal enhancer;

carboxyl-containing fructan such as carboxymethyl inulin; and

a compound selected from the group consisting of: sodium gluconate;gluconic acid; glucono-delta-lactone; sodium gluconate; calciumgluconate; potassium gluconate; and combinations thereof.

Preferably, the scale removal enhancer is selected from the groupconsisting of: potassium carbonate; potassium formate; cesium formateand cesium carbonate and combinations thereof. Preferably, the scaleremoval enhancer is present in the composition in an amount ranging from5 to 20% wt of the weight of the composition. More preferably, the scaleremoval enhancer is present in the composition in an amount ranging from10 to 15% wt of the weight of the composition. Even more preferably, thescale removal enhancer is present in the composition in an amount ofapproximately 10 wt % of the weight of the composition. More preferably,the scale removal enhancer is selected from the group consisting of:K₅DTPA; Cs₅DTPA; Na₄EDTA; and K₄EDTA.

According to a preferred embodiment of the present invention, thechelating agent and counterion are present in the composition in anamount ranging from 5 to 40 wt % of the weight of the composition. Morepreferably, the chelating agent and counterion are present in thecomposition in an amount ranging from 10 to 30 wt % of the weight of thecomposition. Even more preferably, the chelating agent and counterionare present in the composition in an amount ranging from 10 to 20 wt %of the weight of the composition.

According to a preferred embodiment of the present invention, the pH ofthe composition ranges from 10 to 11.

According to a preferred embodiment of the present invention, thecarboxymethyl inulin is present in the composition in an amount rangingfrom 0.5 to 15% wt of the weight of the composition.

According to a preferred embodiment of the present invention, the sodiumgluconate is present in the composition in an amount ranging from 1% to20% wt of the weight of the composition.

According to an aspect of the present invention, there is provided amethod of removing calcium sulfate anhydrate scale present on acontaminated surface, said method comprising:

providing a liquid composition comprising:

a chelating agent selected from the group consisting of: Li₅DTPA;Na₅DTPA; K₅DTPA; Cs₅DTPA; Na₄EDTA; K₄EDTA; TEAH₄DTPA; and TBAH₅DTPA;

optionally, a scale removal enhancer;

carboxymethyl inulin; and

a compound selected from the group consisting of: sodium gluconate;gluconic acid; glucono-delta-lactone; sodium gluconate; calciumgluconate; potassium gluconate;

exposing said surface contaminated with said mixed sulfate scale to theliquid composition;

allowing sufficient time of exposure to remove said calcium sulfateanhydrate scale from the contaminated surface.

Preferably, the composition comprises: a chelating agent selected fromthe group consisting of: Li₅DTPA; Na₅DTPA; K₅DTPA; Cs₅DTPA; Na₄EDTA;K₄EDTA; TEAH₄DTPA; and TBAH₅DTPA; optionally, a scale removal enhancer;carboxymethyl inulin; and sodium gluconate. Preferably, the scaleremoval enhancer is selected from the group consisting of: potassiumcarbonate; potassium formate; cesium formate and cesium carbonate andcombinations thereof.

According to an aspect of the present invention, there is provided amethod of solubilizing barium sulfate into particles of less than 1micron in size, said method comprising:

providing a surface contaminated with a scale containing barium sulfate;

providing a liquid composition comprising:

a chelating agent selected from the group consisting of: Li₅DTPA;Na₅DTPA; K₅DTPA; Cs₅DTPA; Na₄EDTA; K₄EDTA; TEAH₄DTPA; and TBAH₅DTPA;

optionally, a scale removal enhancer;

a carboxyl-containing fructan such as carboxymethyl inulin; and

exposing said surface contaminated with said barium sulfate scale to theliquid composition;

allowing sufficient time of exposure to remove particles of bariumsulfate scale from the contaminated surface;

wherein said particles of barium sulfate complexted with thecarboxymethyl inulin and have a particle size of less than 1 micron;

allowing the pH of the solution to drop from a pH ranging from 10 to 11to a pH ranging from 7 to 8 thereby causing a reprecipitation of thesolubilzed barium sulfate to a particle size of less than 1 micron.

Preferably, the scale removal enhancer is selected from the groupconsisting of: potassium carbonate; potassium formate; Cs₂COOH; Cs₂CO₃;and combinations thereof. Preferably, the carboxyl-containing fructan isa derivative of inulin or another fructan that contains 0.3-3 carboxylgroups per anhydrofructose unit. According to a preferred embodiment ofthe present invention, the derivative of inulin or another fructan thatcontains 0.3-3 carboxyl groups per anhydrofructose unit contains atleast 0.8 carboxyl groups per anhydrofructose unit. Most preferably, thecarboxyl-containing fructan is carboxymethylinulin (CMI).

According to a first aspect of the present invention, there is providedan aqueous composition for use in removing mixed sulfate scale from asurface contaminated with such, said composition comprising:

a chelating agent and a counterion component selected from the groupconsisting of: Li₅DTPA; Na₅DTPA; K₅DTPA; Cs₅DTPA; Na₄EDTA; K₄EDTA;TEAH₄DTPA; and TBAH₅DTPA;

a scale removal enhancer;

sodium gluconate or the like; and

carboxymethyl inulin.

According to another aspect of the present invention, there is provideda method of removing mixed sulfate scale, said method comprising thesteps of:

providing a liquid composition comprising:

a chelating agent selected from the group consisting of: Li₅DTPA;Na₅DTPA; K₅DTPA; Cs₅DTPA; Na₄EDTA; K₄EDTA; TEAH₄DTPA; and TBAH₅DTPA;

a scale removal enhancer;

sodium gluconate or the like; and

carboxymethyl inulin;

exposing a surface contaminated with mixed sulfate scale to the liquidcomposition;

allowing sufficient time of exposure to remove the mixed sulfate scalefrom the contaminated surface.

According to another aspect of the present invention, there is providedan aqueous composition for use in removing mixed sulfate scale from asurface contaminated with such, said composition comprising:

a chelating agent and a counterion component selected from the groupconsisting of: Li₅DTPA; Na₅DTPA; K₅DTPA; Cs₅DTPA; Na₄EDTA; K₄EDTA;TEAH₄DTPA; and TBAH₅DTPA;

a scale removal enhancer;

sodium gluconate or the like; and

carboxymethyl inulin.

Preferably, the scale removal enhancer is selected from the groupconsisting of: potassium carbonate; potassium formate; cesium formateand cesium carbonate and combinations thereof. Preferably, the scaleremoval enhancer is present in the composition in an amount ranging from5 to 20% wt of the weight of the composition. More preferably, from 10to 15% wt of the weight of the composition. Also preferably, the scaleremoval enhancer is present in the composition in an amount ofapproximately 10% wt of the weight of the composition.

Preferably, the chelating agent and counterion are present in thecomposition in an amount ranging from 5 to 40% wt of the weight of thecomposition. More preferably, from 10 to 30% wt of the weight of thecomposition. Also preferably, the chelating agent and counterion arepresent in the composition in an amount ranging from 10 to 20% wt of theweight of the composition.

Preferably, the pH of the composition ranges from 10 to 11.5. Morepreferably, the composition has a pH ranging from 10 to 11.

BRIEF DESCRIPTION OF THE FIGURE

The invention may be more completely understood in consideration of thefollowing description of various embodiments of the invention inconnection with the accompanying figures, in which:

FIG. 1 is the graphical representation of the dissolution performance ofa composition according to a preferred embodiment of the presentinvention in comparison to various other commercially available sulfatescale dissolver over a time period of 24 hours at 60° C.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

By the addition of potassium carbonate to K₅DTPA, the same solubilitynumbers can be attained at a lower pH. Instead of 13.5 a pH of 11 wassufficient to get comparable solubility numbers. This represents aconsiderable difference to typical commercially available products. Thisallows an operator to conduct scale removal operations at a lower pH andtherefore increases the safety for the personnel handling the remover oranyone in the surrounding area as well as environmental risks and cleanup costs in the case of an uncontrolled release.

According to a preferred embodiment of the present invention, the mixedsulfate scale removing composition provides greatly improved rates ofscale dissolution. This, in turn, reduces the down time for wells wherethe scale is being removed and associated the costs. It also reduces thecost of such treatment, by limiting the treatment time by allowingproduction to recommence.

As shown below, the compositions tested for removing non-contaminatedbarium sulfate scale permits the removal thereof at a much lower pH thanwhat has been practiced to date. Indeed, such a composition caneffectively remove barium scale under conditions where the pH is 11,rather than other scale removal compositions which require conditionswhere the pH is 13 or higher. Accordingly, a preferred compositionaccording to the present invention may remove, at pH=10 up to 30 kg/m³of non-contaminated BaSO₄ scale. When using the term “non-contaminatedBaSO₄ scale”, it should be understood to the person skilled in the art,that what is meant is that the barium sulfate scale is not contaminatedby petroleum product or a petroleum-based product.

According to a preferred embodiment of the present invention, acomposition for removing mixed sulfate scale permits the removal thereofwith a higher dissolution capacity. This, in turn, allows reducing thevolume of scale remover necessary. This also decreases transport costsand many other related items resulting from the usage of lower volumesof scale remover.

According to a preferred embodiment of the present invention, theadditional sulfate scale dissolver comprising sodium gluconate or thelike. Compounds having a gluconate component or portion are understoodto fall in the later category but do not comprise the entire category asother sugars are also considered to be within this description.Preferably, the compounds having a gluconate component or portioninclude but are not limited to: gluconic acid (CAS #526-95-4);glucono-delta-lactone (CAS #90-80-2); sodium gluconate (CAS #527-07-1);calcium gluconate (CAS #299-28-5/18016-24-5); potassium gluconate (CAS#299-27-4). Potassium gluconate being preferred.

According to a preferred embodiment of the present invention, thecarboxyl-containing fructans are understood to be a derivative of inulinor another fructan that contains 0.3-3 carboxyl groups peranhydrofructose unit. In particular, the derivative contains at least0.8 carboxyl groups per anhydrofructose unit. Preferably, the carboxylgroups can be present in the form of carboxyalkyl groups, such ascarboxymethyl, carboxyethyl, dicarboxymethyl or carboxyethoxycarbonylgroups. According to a preferred embodiment of the present invention,mixed carboxyfructans can also be used. Preferably, the number ofcarboxymethyl groups is greater than the number of other carboxylgroups. The most preferred of the carboxyl-containing fructans iscarboxymethylinulin (CMI). Carboxymethylinulin (CMI) having a degree ofsubstitution of of 0.15-2.5 is disclosed in WO 95/15984. Mixed carboxylderivatives the inulin can have been first carboxymethylated and thenoxidised or vice versa. According to a preferred embodiment of thepresent invention, the carboxyl-containing fructan has an average chainlength (=degree of polymerisation, DP) ranging between 3 and 1000, butpreferably, the average chain length ranges from 6-60 monosaccharideunits.

Absolute Solubility of Barium Sulfate Scale

The inventors have previously noted that chelating agents such as EDTA(Ethylenediaminetetraacetic acid) or DTPA (diethylenetriaminepentaaceticacid) and the ability to dissolve non-contaminated barium sulfatedepends substantially on the size and ion strength of the counterion.

In Tables 1 and 2 (absolute solubility testing) the absolute (ormaximum) solubility of non-contaminated increases with the size of thecounterion from lithium to cesium. TEAH (Tetraethylammonium hydroxide)and TBAH (Tetrabuthylammonium hydroxide) as organic bases (counterions)are showing the same trend. Information indicates that the size of theTBAH cation (including the hydrate layer) is comparable to potassium.

The solubility numbers for both were found to be very similar. In orderto quantitatively compare the kg/solubility properly, the BaSO₄:chelating agent ratio was calculated in g/mol and the Ba²⁺:chelatingagent ratio was calculated in mol/mol. The mol:mol ratio indicates thenumber of molecules of the chelating agent needed to dissolve one ion ofBa²⁺ (complex). The highest ratio which was found was almost 0.5, whichmeans that there needs to be, on average, 2 molecules of DTPA todissolve 1 Ba²⁺ ion but mostly it can be much less.

Tests performed have indicated that, besides the nature of thecounterion, an excess of the counterion also improves the solubility.K₅DTPA was tested in conjunction with KCl, K₂CO₃ and KOOCH (potassiumformate). It seems that the counterion play also a large role as K₂CO₃(with the larger anion) was much more effective than KCl (with a smallanion).

TABLE 1 Absolute solubility of non-contaminated barium sulfate scale(when using a 40% solution of the scale removing composition) 40wt % solBaSO4 BaSO4 Ba2⁺ pH (kg/m3) (g/mol) (mol/mol) Li₅DTPA 2 Na₅DTPA 13.01 1720.24 0.088 K₅DTPA 13.25 46 62.16 0.266 K₅DTPA + 13.21 38 51.35 0.2210wt % K₂CO₃ Cs₅DTPA 13.4 52 72.2 0.309 Na₄EDTA 13.11 9 7.89 0.034K₄EDTA 13.32 31 32.98 0.141 TEAH₄DTPA 13.1 14 43.75 0.187 TBAH₅DTPA13.33 18 64.28 0.275

TABLE 2 Absolute solubility of non-contaminated barium sulfate scale(when using a 20% solution of the scale removing composition) at 60° C.20wt % sol BaSO4 Ba2⁺ pH BaSO4 (kg/m3) (g/mol) (mol/mol) K₅DTPA 13.19 2772.97 0.313 K₅DTPA + 13.32 41 110.81 0.475 5 wt % K₂CO₃ K₅DTPA + 11.2540 108.11 0.463 5 wt % K₂CO₃ K₅DTPA + 10 33 89.19 0.3821 5 wt % K₂CO₃Cs5DTPA + 35 5 wt % CsCO3 Cs5DTPA + 35 10 wt % CsCO₃ Cs5DTPA + 30 10 wt% HCOOCs TEAH4DTPA + 21 10 wt % K₂CO₃ TBAH5DTPA + 25 10 wt % K₂CO₃

Moreover, the K₅DTPA composition (at 40%) was determined to dissolve 30kg/m³ of FeS for a g/mol total of 40.54.

Preferably, the dissolution of non-contaminated barium sulfate in anamount above 20 kg/m³. More preferably, dissolution of barium sulfateabove 30 kg/m³ is desired.

Speed of Barium Scale Dissolution

A second set of tests were performed to study the speed of dissolutionof non-contaminated barium sulfate scale. In order to determine thespeed, a relatively small amount of BaSO₄ (0.25 g—this equates to 10kg/m³) was used and the time was measured until the solution becameclear. Large differences were noted. The best results involved thecombination of K₅DTPA with K₂CO₃. This combination provided adissolution time which was almost 4 times faster than K₅DTPA alone.

The speed of dissolution of compositions according to preferredembodiment of the present invention were tested and studied. Table 3summarizes the findings of the testing. The experiment involved thedissolution of 0.25 g of BaSO₄ in a volume of 50 ml fluid at 60° C.under gentle stirring by magnetic stir bar.

TABLE 3 Speed of dissolution of non-contaminated barium sulfate scaleFluid Time pH K₅DTPA (40%) 1 h 44 min 13.26 K₅DTPA (40%) + 10% TBAH 1 h38 min 13.4 K₅DTPA (40%) + 20% TBAH 1 h 21 min 13.43 K₅DTPA (40%) + 30%TBAH 1 h 20 min 13.49 K₅DTPA (40%) + 10 wt % KCl 1 h 24 min 13.27 K₅DTPA(40%) + 10% K₂CO₃   30 min 13.22 K₅DTPA (20%) + 5% K₂CO₃ 22-23 min10.5-11

This testing indicates that both the extent of barium scale dissolutionand the speed at which it is dissolved represent marked improvementsover known compositions.

Preferably, the scale removal enhancer is selected from the groupconsisting of: K₂CO₃; KOOCH; CsCO₃; CsCOOH and combinations thereof.Preferably, the scale removal enhancer is K₂CO₃. Preferably also, thescale removal enhancer is present in an amount ranging from 5 to 30% byweight of the scale removal composition. More preferably from 10 to 20%by weight and even more preferably, the scale removal enhancer would bepresent in an amount of approximately 10% by weight.

Impact of Temperature

The speed of dissolution of a barium scale dissolver composition wastested and studied under different temperature conditions onnon-contaminated barium sulfate scale. Table 4 summarizes the findingsof the testing. The experiment involved the dissolution of 0.25 g ofBaSO₄ in a volume of 50 ml fluid at various temperatures under gentlestirring by magnetic stir bar. The composition tested comprised a 20 wt% solution of K₅DTPA and 5 wt % K₂CO₃.

TABLE 4 Impact of Temperature on the Dissolution of Barium SulfateTemperature Time in ° C. (° F.) (minutes) 25 (77)  225 40 (104) 50 60(140) 22 80 (176) 3.5 90 (194) 1.5

Laboratory Testing of Scale Dissolution

The sample selected for the solubility testing origins from an oilfieldtubular containing sulfate scale crystals originally used fordemonstration purposes. Crystals of non-contaminated barium sulfatescale were removed from the tubular to be used for the solubilitytesting. 200 cc of composition (K₅DTPA 20 wt % and 5 wt % K₂CO₃) wasused. A weighted portion of oilfield sulfate scale sample was submergedin 200 cc of each de-scaling composition. A small magnetic stirrer isadded to create a very minimal vortex, creating a small movement offluid without rigorously stirring the fluid. The fluid was heated to 70°Celsius.

Results

25.165 grams of non-contaminated oilfield sulfate scale was weighted andadded to the fluid. The stirrer and heater were started. After 1 hour aslight colouring of the fluid was observed. After 4 hours at temperaturewhen no continued visual reduction of scale was observed, the fluid hasbeen filtered and the filter rinsed with water, dried and weighted back.The maximum scale solubility was reached and subsequently calculated.

The base barium scale dissolver composition (used in later testing andreferred to as “base BSD”) comprises a 20 wt % solution of K₅DTPA and 5wt % K₂CO₃. The base BSD was able to dissolve 52.97 grams per litre ofscale at 70° C. The testing was also carried out with a commerciallyavailable product (Barsol NS™), which is alkali/EDTA based and withEDTA. The Barsol NS™ product was capable of dissolving 24.19 grams perlitre. While EDTA alone only dissolved around 6 grams per litre. Underidentical conditions, the base BSD was shown to have more than doublethe performance of Barsol NS™.

According to a preferred embodiment of the present invention, there isprovided a one—step process for removing mixed sulfate scale inside awellbore, said process comprising:

providing a liquid composition comprising:

a chelating agent selected from the group consisting of: Li₅DTPA;Na₅DTPA; K₅DTPA; K₅DTPA; Cs₅DTPA; Na₄EDTA; K₄EDTA; TEAH₄DTPA; andTBAH₅DTPA;

a carboxyl-containing fructan or a salt thereof such as carboxymethylinulin; and

a compound selected from the group consisting of: sodium gluconate;gluconic acid; glucono-delta-lactone; sodium gluconate; calciumgluconate; potassium gluconate;

exposing a surface contaminated with mixed sulfate scale to the liquidcomposition;

allowing sufficient time of exposure to remove some or all of the mixedsulfate scale from the contaminated surface. The person skilled in theart will understand that what is meant by “one-step” is that there is asingle treatment step in the process (or method) to remove mixed sulfatescale.

When the surface contaminated with mixed sulfate scale is deepundergound or a hard to access tubing or piping, the exposure consistsof circulating the liquid composition through the tubing or piping untilit has been established that the scale has been removed beyond adesirable predetermined point. Hence, in some cases, it is quitepossible that the entirety of the scale present is not removed but theamount of removal is sufficient to re-start operations and provide thedesired productivity and/or circulation through the affectedtubing/piping. The liquid composition can also be heated in order toimprove the removal of the scale and the speed at which the removal iseffected.

According to another preferred embodiment of the present invention, themethod of treatment of mixed sulfate scale wherein the fluid is«spotted», i.e. placed in a tube/tank/pipe/equipment in a soakingoperation. This may in some instances be somewhat less efficient thancirculating or agitating the fluid due to the surface reaction nature ofthe fluid, but it is used in some cases to remove enough scale to runtools, pull stuck tubing or free blocked flow control equipment etc.,for example.

Sulfate scales that are commonly found inside wellbores include calciumsulfate, strontium sulfate and barium sulfate. Up to now, it wasbelieved that an effective barium sulfate scale dissolver was themissing link in order to remove very difficult to remove scales. It wassurprisingly discovered that depending on the type of calcium sulfatepresent in the mixed sulfate scales the scales may be more or less easyto remove. The most common form of calcium sulfate is the dihydrate.Upon exposure to pressure and temperature, the dihydrate converts tohemihydrate and ultimately to the anhydrous form. Calcium sulfateanhydrous presents a substantially more difficult to remove scale thanits dihydrate counterpart.

Tests were carried out in order to assess the advantage of a compositionaccording to a preferred embodiment versus a typical mix of sulfatescales that is encountered during oil industry operations. As calciumsulfate is by far the most common scale component, it is believed thatthese tests are quite representative of actual mixture. The solubilityof the scale mixture was evaluated against the base BSD composition andtwo preferred embodiments of the present invention. The results arelisted in Table 5 below.

TABLE 5 Solubility of mixed sulfate scales comprising Calcium sulfate,Strontium sulfate and barium sulfate at 60° C. Sodium 25-30 wt of wt ofwt of fil Total Gluconate UP scale filter and prod Solubility Solution(wt %) (vol %) Scale (g) (g) (g) kg/m3 100% base BSD  5 1 80% CaSO4•2H2O10.0003 0.2764 3.1851 70.916 1% SrSO4 19% BaSO4 100% base BSD 10 1 80%CaSO4•2H2O 10.0006 0.2872 3.1888 70.990 1% SrSO4 19% BaSO4 100% base BSD\ \ 80% CaSO4•2H2O 10.0017 0.2904 3.6249 66.672 1% SrSO4 19% BaSO4 80%base BSD, 10 1 80% CaSO4•2H2O 10.0048 0.2829 3.9884 62.993 20% H2O 1%SrSO4 19% BaSO4 NB: 100% base BSD refers to an undiluted solution of thecomposition as set out previously. A diluted solution of base BSD isreferred to in the amount of residual stock concentration afterdilution. 25-30 UP refers to a commercial sodium carboxymethyl inulincomposition having a 30-32 wt % NaCMI content.

Tests were carried out in order to assess the advantage of a compositionaccording to a preferred embodiment versus a calcium scale in both thedihydrate form and in the anhydrous (anhydrate) form. As calcium sulfateis the most common scale component in a mixture of sulfate scales, itwas believed to be important to assess the effectiveness of the knowndescaler against two preferred embodiments of the present invention. Itis known that while calcium sulfate dihydrate and calcium sulfateanyhdrate have greatly different properties and that the dihydrate formis both the most common and first to be formed when depositing. What isalso known is that the dihydrate will convert to more stable forms uponexposure to heat and pressure. The most stable form of calcium sulfatebeing the anyhydrate. The results of the experiments are listed in Table6 below.

TABLE 6 Solubility of Calcium sulfate (anhydrous) and Calcium sulfate(dihydrate) wt of Sodium 25-30 wt of wt of fil and Total Gluconate UPscale filter prod Solubility Solution (wt %) (vol %) Scale (g) (g) (g)kg/m3 100% Base BSD 5 1 100% CaSO4•2H2O 10.0001 0.2773 3.1126 71.648100% Base BSD 10  1 100% CaSO4•2H2O 10.0021 0.2653 2.7927 74.747 100%Base BSD \ \ 100% CaSO4•2H2O 10.0009 0.2647 3.0829 71.827 100% Base BSD5 1 100% CaSO4 (Anhydrous) 10.0027 0.2873 5.8483 44.417 100% Base BSD10  1 100% CaSO4 (Anhydrous) 10.0007 0.2774 5.4992 47.789 100% Base BSD\ \ 100% CaSO4 (Anhydrous) 10.0058 0.2774 7.9699 23.133 100% Base BSD \\ 100% CaSO4 (Anhydrous) 10.0017 0.2662 7.1513 31.166

The results of Table 6 indicate that the type of calcium scale (anydratevs dihydrate) has a substantial and marked impact of the dissolutionefficiency of the scale dissolvers tested.

The use of sodium gluconate is an effective component in the removal ofsmaller cations present in a mixed sulfate scale. Varying the amount ofsodium gluconate (or the like) can have a direct impact on theeffectiveness of the composition according to a preferred embodiment ofthe present invention as it provides for an increased dissolution powerof mixed sulfates scale. Sodium gluconate is a representative compoundof sugars which have the same properties including but not limited tothe gluconate or the like categorization and it is quite effective inthe presence of scale containing transition metal cations (such as, butnot limited to, iron, manganese, zinc, tin) and post-transition metalcations (such as, but not limited to, aluminum, lead) as these cationshave typically a smaller ion radii. According to a preferred embodiment,the gluconate or the like can be present in a concentration ranging from0.1 wt % to 20 wt % of the total weight of the composition, morepreferably from 1 to 20 wt %. According to another preferred embodiment,the gluconate or the like can be present in a concentration ranging from1 wt % to l0 wt % of the total weight of the composition, morepreferably ranging from 1 to 5 wt %, even more preferably ranging from 1to 3 wt %.

While the presence of sodium gluconate or the like is preferable in somecases, the presence of such a compound has its limitations. Foroperations carried out at temperatures of 150° C. or more, gluconateshave a tendency of being less stable and degrading, and thus would benot desirable for such applications. For this reason, its use in hightemperature applications has limitations. Hence, in situations where theoperating temperature encountered by the compositions used are of 150°C. or higher, a preferred composition of the present invention will notnecessarily require the presence of a gluconate compound (or the like).Thus, a preferred embodiment of the present invention to be used for theremoval of mixed sulfate scale at high temperatures (i.e. above 150° C.)will comprise: a chelating agent selected from the group consisting of:Li₅DTPA; Na₅DTPA; K₅DTPA; Cs₅DTPA; Na₄EDTA; K₄EDTA; TEAH₄DTPA; andTBAH₅DTPA; optionally, a scale removal enhancer; a carboxyl-containingfructan such as carboxymethyl inulin. Thus, a preferred embodiment ofthe present invention to be used for the removal of predominantly bariumsulfate scale will comprise: a chelating agent selected from the groupconsisting of: Li₅DTPA; Na₅DTPA; K₅DTPA; Cs₅DTPA; Na₄EDTA; K₄EDTA;TEAH₄DTPA; and TBAH₅DTPA; optionally, a scale removal enhancer; acarboxyl-containing fructan such as carboxymethyl inulin.

Moreover, the compositions according to preferred embodiments of thepresent invention used are environmentally safer than many otherdissolvers. This represents a major advantage over any knownchemically-based methods of mixed sulfate scale. Another advantage tothe compositions according to preferred embodiments of the presentinvention includes the speed of dissolution which is considerably fasterthan any known commercial compositions. Another advantage of preferredcompositions according to the present invention is that they can beemployed on wells according to a one-step process and thus are verydesirable to operators which deal with mixed sulfate scale issues on aregular basis, such as in the North Sea.

According to another aspect of the present invention, there is provideda method of solubilizing barium sulfate into particles of less than 1micron in size, said method comprising:

providing a surface contaminated with a scale containing barium sulfate;

providing a liquid composition comprising:

a chelating agent selected from the group consisting of: Li₅DTPA;Na₅DTPA; K₅DTPA; Cs₅DTPA; Na₄EDTA; K₄EDTA; TEAH₄DTPA; and TBAH₅DTPA;

optionally, a scale removal enhancer;

a carboxyl-containing fructan such as carboxymethyl inulin; and

a compound selected from the group consisting of: sodium gluconate;gluconic acid; glucono-delta-lactone; sodium gluconate; calciumgluconate; potassium gluconate;

exposing said surface contaminated with said barium sulfate scale to theliquid composition;

allowing sufficient time of exposure to remove particles of bariumsulfate scale from the contaminated surface; wherein said particles ofbarium sulfate are complexed with the carboxymethyl inulin and have aparticle size of less than 1 micron;

allowing the pH of the solution to drop from a pH ranging from 10 to 11to a pH ranging from 7 to 8 thereby causing a reprecipitation of thesolubilzed barium sulfate to a particle size of less than 1 micron (dueto the low solubility product of barium sulfate at low pH (7 to 8).

The interaction of the CMI with the dissolved barium sulfate is notcompletely clear but it seems that the CMI interacts/intereferes withthe crystal surface of the barium sulfate in order toprevent/minimize/inhibit crystal growth and thus maintain barium sulfateat a particle size of less than 1 micron. It is hypothesized that theCMI-barium sulfate complex creates a sort of nanoparticle which, becauseof its small size is capable of undergoing brownian motion and thusnever quite settling (i.e. does not reprecipitate, at least during theperiod of duration (up to 7 days) which the testing herein seems tosupport). According to a preferred embodiment, the carboxyl-containingfructan can be present in a concentration ranging from 0.01 wt % to 15wt % of the weight of the composition, more preferably from 0.5 wt % to15 wt %. According to another preferred embodiment, thecarboxyl-containing fructan can be present in a concentration rangingfrom 0.01 wt % to 1 wt %. According to a more preferred embodiment, thecarboxyl-containing fructan can be present in a concentration rangingfrom 0. 1 wt % to 0.5 wt %. According to yet another preferredembodiment, the carboxyl-containing fructan can be present in aconcentration ranging from 0.01 wt % to 0.4 wt %, more preferably from0.1 wt % to 0.35 wt %, even more preferably from 0.25 wt % to 0.32 wt %

Once a composition according to a preferred embodiment of the present isexposed to a surface contaminated with sulfate scale, the scale isremoved over a period of time but the dissolved scale is at risk ofreprecipitating upon exposure to formation water. Since the scaledissolver has a pH preferably ranging from 11 to 11.5, the bariumsulfate and other scales will dissolve but as the dissolved scale isincreasingly exposed to the formation water which typically has a pH ofabout 6 to 7, the pH of the water around the dissolved scale willdecrease. The reprecipitation of barium sulfate at around pH=8 isunavoidable as the ksp for barium sulfate is very low at such pH. Thisis one of the main reasons why the above mentioned preferred compositionis desirable as it will prevent reprecipitation of barium sulfate andthus allow fluids to flow after scale removal.

Comparative Testing of a Preferred Scale Dissolver of the PresentInvention

In order to assess the efficiency of a preferred scale dissolver of thepresent invention, it was compared to three other commercially bariumsulfate scale dissolver at 50° C. and at 90° C. The results of thetestings are found in tables 7 and 8 below. Compositions A, B, and C arecommercially available barium sulfate scale dissolvers. Composition D isa preferred scale dissolver of the present invention which comprises thebase BSD+10 wt % sodium gluconate and 1% vol of CMI (at approximately30-32 wt %).

TABLE &num;7 Amounf of barium sulfate scale dissolved after 24 hours ata temperature of 50° C. Barium Sulfate Scale dissolved Composition (inwt %) A 57 B 16 C 72 D 84

TABLE &num;8 Amounf of barium sulfate scale dissolved after 24 hours ata temperature of 90° C. Barium Sulfate Scale dissolved Composition (inwt %) A 77 B 22 C 79 D 87

In both experiments, the composition according to a preferred embodimentof the present invention (Composition D) performed better than all threecommercially available barium sulfate scale dissolvers. Anothernon-negligible observation is that all three commercially availablebarium sulfate scale dissolvers (A, B and C) have a pH above 12 (someclose to 13), while Composition D has a pH ranging between 11 and 11.5.This difference in pH is significant for operators and any personnelhandling this type of caustic product, and thus it is highly desirableto have a product with a pH as close to neutral as possible.

Comparative Testing with Other Sulfate Scale Inhibitors

In another round of testing, various commercially available bariumsulfate scale dissolvers (compositions A, B, D, E, F, I, J, K, L, N, andP) were tested for mixed sulfate scale dissolving efficiency (at 60° C.)compared to a composition according to a preferred embodiment of thepresent invention (Composition Q which comprises base BSD+CMI (1% vol.(at approximately 30-32 wt %))+sodium gluconate (10 wt %)).

Composition Q performed better than all of the commercially availablemixed sulfate scale dissolvers. The composition of the scale was (majorelements only): barium (44 wt %); calcium (4.4%); strontium (8.4 wt %),the balance of the composition of the scale being mainly made up by theanions and organic compounds. The scale dissolution for each compositionwas measured at 2 hours, 4 hours 8 hours and 24 hours after start oftreatment. FIG. 1 is the graphical representation of the performance ofeach composition over a time period of 24 hours at 60° C. The time axis(x-axis) was not graphed to scale as the sheer number of compositionstested would not be easily distinguishable in the first threemeasurements (2, 4 and 8 hours) and the graphical depiction provided inFIG. 1 allows to better assess the efficiency of each composition overtime. The composition according to a preferred embodiment of the presentinvention was by far by the most effective scale dissolver in totaldissolved scale (measured in ppm). As well, contrary to Compositions Aand B which started to have reprecipitation occur after 8 hours,Composition Q maintained the dissolved scale and managed to dissolveeven more scale after the 8 hours measurement.

Barium Sulfate Reprecipitation Laboratory Tests

Several experiments were conducted to assess the ability of certaincompositions (base BSD, base BSD+CMI (1% vol. (at approximately 30-32 wt%)), and base BSD+sodium gluconate (10 wt %)) to maintain the dissolvedbarium sulfate in solution.

Reprecipitation Experiment #1

Three blends containing the Base BSD composition (Base BSD, BaseBSD+CMI, and Base BSD+sodium gluconate) were prepared with added bariumsulfate and were observed as reprecipitation occurred at each pHinterval from 11 to 7 with the addition of 1 N hydrochloric acid. 61.66%of the barium sulfate precipitate was filtered out from the BaseBSD+sodium gluconate solution and 12.94% of the barium sulfateprecipitate was filtered from the Base BSD+CMI solution, with theremainder staying suspended in solution.

Procedure: To observe the reprecipitation of barium sulfate, 2.0000 g ofbarium sulfate was dissolved in 100 mL of each Base BSD compositions(Base BSD, Base BSD+CMI, and Base BSD+sodium gluconate) at 60° C. for 4hours on a heated stir plate at 190 rpm. The solutions were then cooledto ambient temperature, transferred into new beakers and then placedonto stir plates with stir bars. A pH probe was placed in the solutionto monitor the pH of the solution as 1N hydrochloric acid (HCl) wasadded drop wise, with a photo taken at each pH interval.

After 3 days, the reprecipitated solutions were filtered through P8 andthen P2 filter paper and then re-examined after 7 days.

Results and Observations:

Using P8 and P2 filter papers, 61.66% of the barium sulfate precipitatewas filtered out from the Base BSD+sodium gluconate solution. For thebase BSD+CMI solution, 12.94% of the barium sulfate precipitate wasfiltered out while the remaining barium sulfate stayed suspended insolution for a period of up to at least 7 days.

Barium Sulfate Reprecipitation Laboratory Tests—Experiment #2

Base BSD+CMI test solutions were heat treated in a high pressure/hightemperature Teflon lined cell at 150° C. (302° F.), at 400 psi, for 6hours and 24 hours to simulate downhole conditions. Barium sulfate wasthen dissolved in the heat-treated Base BSD+CMI to ensure unalteredfunctionality. Afterwards, the pH was lowered stepwise to 7 with theaddition of 1 N hydrochloric acid. This was executed to determine the pHwhere reprecipitation occurs and if the specific additive in BaseBSD+CMI is still functional after the heat treatment to suppress theprecipitation formation to a lower pH and if the formed barium sulfatecrystal size is still altered to smaller particle sizes by Base BSD+CMI.The different solutions with the reprecipitation were filtered throughfilters with different pore sizes. It was determined that a minimum of94 wt % of the barium sulfate reprecipitate from both Base BSD+CMI testsolutions (6 hours and 24 hours at 150° C.) is smaller than 1 μm. Theremaining crystal material stays suspended in solution for an extendedtime (7 days) without further settling out.

Procedure: Base BSD+CMI solutions were prepared and exposed to a heattreatment in high pressure/high temperature Teflon lined cells at 150°C., at 400 psi, for 6 hours and 24 hours. After the heat treatment, eachcell was depressurized and the solution was allowed to cool to ambienttemperature. No visual decomposition of the additive package in the BaseBSD+CMI was observed.

To determine the functionality of the heat-treated Base BSD+CMI , 2.0 gof barium sulfate was dissolved in 100 mL of the two Base BSD+CMI blends(heat-treated at 6 hours at 150° C., and 24 hours at 150° C.) at 60° C.for 4 hours on a heated stir plate at 190 rpm. The solubility capabilitywas unaltered compared to non heat-treated Base BSD+CMI . The solutionswere then cooled to ambient temperature. The solutions were transferredinto a beaker with a stir bar and placed on a stir plate. A pH probe wasplaced in the solution to monitor the pH of the solution as INhydrochloric acid (HCl) was added (drop wise). A photo was taken at eachpH interval. The first slight reprecipitation of barium sulfate occursat a pH of 8. Full reprecipitation occurred at a pH of 7. The testsolutions were stored at room temperature for 3 days.

After 3 days, the reprecipitated barium sulfate from the Base BSD+CMIsolutions were filtered through P8 and then through P2 filter paper andallowed to rest for 7 days further days.

Results:

Using the P8 and P2 filter paper, 97.4 wt % of the barium sulfatereprecipitate passed both filters from the Base BSD+CMI (24 hours at150° C.) solution and 94.3 wt % of the barium sulfate reprecipitatepassed both filters from the Base BSD+CMI (6 hours at 150° C.) solution.The remaining material stayed suspended in solution. Based on theseresults and observations the assumption is, that the majority of theformed crystalline material is smaller than 1 μm. This is a directeffect of the utilized additive package of the Base BSD+CMI.

Barium Sulfate Reprecipitation Laboratory Tests—Experiment #3

To observe the reprecipitation of barium Sulfate laboratory tests werecarried out to compare three compositions (the base BSD, base BSD+CMI,and base BSD+sodium gluconate) were prepared with barium sulfate to beobserved as reprecipitation occurs at each pH interval from 11 to 7 withthe addition of 1 N hydrochloric acid.

Procedure: To observe the reprecipitation of barium sulfate, 2.0000 g ofbarium sulfate was dissolved in 100 mL of the three tested BSDcompositions (base BSD, base BSD+CMI, and base BSD+sodium gluconate) at60° C. for 4 hours on a heated stir plate at 190 rpm. The solutions werethen cooled to ambient temperature. The solutions were transferred intoa beaker with a stir bar and placed on a stir plate. A pH probe wasplaced in the solution to monitor the pH of the solution as 1Nhydrochloric acid (HCl) was added (drop wise). A photo was taken at eachpH interval.

After 3 days, the reprecipitated solutions were filtered through P8filter paper and then through P2 filter paper and allowed to rest for 7days for further observation. Fischer Scientific P8 filter papers have aporosity such that particles greater than 20 microns are filtered out.The P2 filter paper (from Fischer Scientific) has a porosity such thatparticles as small as 1 micron are filtered out of solution.

Observations: The barium sulfate precipitate was filtered out from thesolution containing only the Base BSD however the precipitate in thesolutions containing the Base BSD+1% vol. CMI and the Base BSD+sodiumgluconate could not be filtered and remained suspended in solution evenafter 7 days.

While the foregoing invention has been described in some detail forpurposes of clarity and understanding, it will be appreciated by thoseskilled in the relevant arts, once they have been made familiar withthis disclosure that various changes in form and detail can be madewithout departing from the true scope of the invention in the appendedclaims.

1. A method of removing mixed sulfate scale, said method comprising:providing a liquid composition comprising: a chelating agent selectedfrom the group consisting of: Li₅DTPA; Na₅DTPA; K₅DTPA; Cs₅DTPA;Na₄EDTA; K₄EDTA; TEAH₄DTPA; and TBAH₅DTPA; optionally, a scale removalenhancer; a carboxyl-containing fructan; and a compound selected fromthe group consisting of: sodium gluconate; gluconic acid;glucono-delta-lactone; sodium gluconate; calcium gluconate; potassiumgluconate; exposing a surface contaminated with said mixed sulfate scaleto the liquid composition; and allowing sufficient time of exposure toremove said mixed sulfate scale from the contaminated surface.
 2. Themethod according to claim 1, wherein the scale removal enhancer isselected from the group consisting of: potassium carbonate; potassiumformate; cesium formate; cesium carbonate; and combinations thereof. 3.The method according to claim 3, wherein the carboxyl-containing fructanis carboxymethylinulin (CMI).
 4. An aqueous composition for use inremoving mixed sulfate scale from a surface contaminated with such, saidcomposition comprising: a chelating agent and a counterion componentselected from the group consisting of: Li₅DTPA; Na₅DTPA; K₅DTPA;Cs₅DTPA; Na₄EDTA; K₄EDTA; TEAH₄DTPA; and TBAH₅DTPA; optionally, a scaleremoval enhancer; carboxyl-containing fructan; and a compound selectedfrom the group consisting of: sodium gluconate; gluconic acid;glucono-delta-lactone; sodium gluconate; calcium gluconate; potassiumgluconate.
 5. The aqueous composition according to claim 4, wherein thescale removal enhancer is selected from the group consisting of:potassium carbonate; potassium formate; cesium formate and cesiumcarbonate and combinations thereof.
 6. The aqueous composition accordingto claim 4, wherein the scale removal enhancer is present in thecomposition in an amount ranging from 5 to 20% wt of the weight of thecomposition.
 7. The aqueous composition according to claim 4, whereinthe scale removal enhancer is present in the composition in an amountranging from 10 to 15% wt of the weight of the composition.
 8. Theaqueous composition according to claim 4, wherein the scale removalenhancer is present in the composition in an amount of approximately 10wt % of the weight of the composition.
 9. The aqueous compositionaccording to claim 4, wherein the chelating agent and counterion arepresent in the composition in an amount ranging from 5 to 40 wt % of theweight of the composition.
 10. The aqueous composition according toclaim 4, wherein the chelating agent and counterion are present in thecomposition in an amount ranging from 10 to 30 wt % of the weight of thecomposition.
 11. The aqueous composition according to claim 4, whereinthe chelating agent and counterion are present in the composition in anamount ranging from 10 to 20 wt % of the weight of the composition. 12.The aqueous composition according to claim 4, wherein the pH of thecomposition ranges from 10 to
 11. 13. The aqueous composition accordingto claim 4, wherein the scale removal enhancer is selected from thegroup consisting of: K₅DTPA; Cs₅DTPA; Na₄EDTA; and K₄EDTA.
 14. Theaqueous composition according to claim 4, wherein the carboxymethylinulin is present in the composition in an amount ranging from 0.5 to15% wt of the weight of the composition.
 15. The aqueous compositionaccording to claim 4, wherein the sodium gluconate is present in thecomposition in an amount ranging from 1% to 20% wt of the weight of thecomposition.
 16. A method of solubilizing barium sulfate into particlesof less than 1 micron in size, said method comprising: providing asurface contaminated with a scale containing barium sulfate; providing aliquid composition comprising: a chelating agent selected from the groupconsisting of: Li₅DTPA; Na₅DTPA; K₅DTPA; Cs₅DTPA; Na₄EDTA; K₄EDTA;TEAH₄DTPA; and TBAH₅DTPA; optionally, a scale removal enhancer; acarboxyl-containing fructan such as carboxymethyl inulin; and exposingsaid surface contaminated with said barium sulfate scale to the liquidcomposition; allowing sufficient time of exposure to remove particles ofbarium sulfate scale from the contaminated surface; wherein saidparticles of barium sulfate complexted with the carboxymethyl inulin andhave a particle size of less than 1 micron; and allowing the pH of thesolution to drop from a pH ranging from 10 to 11 to a pH ranging from 7to 8 thereby causing a reprecipitation of the solubilzed barium sulfateto a particle size of less than 1 micron.
 17. The method according toclaim 16, wherein the carboxyl-containing fructan is carboxymethylinulin(CMI).
 18. The method according to claim 16 wherein the compositioncomprises: a chelating agent selected from the group consisting of:Li₅DTPA; Na₅DTPA; K₅DTPA; Cs₅DTPA; Na₄EDTA; K₄EDTA; TEAH₄DTPA; andTBAH₅DTPA; optionally, a scale removal enhancer; carboxymethyl inulin;and sodium gluconate
 19. The method according to claim 16 wherein thescale removal enhancer is selected from the group consisting of:potassium carbonate; potassium formate; cesium formate and cesiumcarbonate and combinations thereof.